Mask and method for manufacturing the same

ABSTRACT

A mask and a method for manufacturing the same, which can form a correct pattern on a semiconductor wafer having steps, includes a mask substrate having steps oppositely corresponding to steps on said semiconductor wafer and an opaque mask pattern for cutting off light from the mask substrate to thereby enable the same exposure focus to be provided to the step and non-step regions on the semiconductor wafer. Further, a clean and correct pattern can be formed by controlling the amount of exposure irradiated onto the step and non-step regions on the semiconductor wafer.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor photolithographytechnique, and more particularly, to a photolithography mask and methodfor manufacturing the mask which can be used to form a fine pattern on asemiconductor substrate.

Generally, it is well known that various semiconductor patterns can beformed by photolithography. Photolithography can largely be divided intotwo steps.

First, a photoresist whose solubility is changed by exposure to, forexample, ultraviolet, X-ray or electron beam radiation, is coated on aninsulating film or a conductive film formed on a semiconductorsubstrate, i.e., on a film wherein a pattern is to be formed. Apredetermined portion of the photoresist is exposed to light using amask, and the portion having a high solubility is removed by adevelopment process, to thereby form a photoresist pattern.

Second, an exposed portion of film is removed by an etching process, tothereby form various kinds of patterns such as wiring or electrodes.

Recently, photolithography has become an important process in theproduction of semiconductor devices having a high packing density.Typically, a semiconductor device repeatedly goes through the process ofserially forming and patterning a plurality of films on a semiconductorsubstrate. As the manufacturing procedure has developed, high packingdensity of the semiconductor device can be achieved when the finepattern is formed on a stepped structure.

FIG. 1 shows a method for forming a pattern by the conventional method.

When an ultraviolet or an electron beam 1 is irradiated onto a mask 2having a mask pattern, the mask pattern is projected via a projectionlens 3 of a stepper onto a photoresist formed on a semiconductor wafer 9having a stepped portion 110. At this time, the part of the photoresistformed on the top of stepped portion 110 is sufficiently exposed(reference numeral 4 denotes an unexposed photoresist portion and 5denotes an exposed photoresist portion), whereas the photoresist portion6 formed over the wafer 9 is under-exposed. The resulting photoresistpattern is shown in (B) of FIG. 1. The part of the photoresist formedover the stepped portion 110 is much thinner than that formed over thewafer regions which are below the stepped portion 110, when aphotoresist is coated onto the semiconductor wafer 9 having steppedportion 110. Thus, when the photoresist is exposed, the photoresistformed on the wafer regions at the bottom of stepped portion 110 isinsufficiently exposed. Therefore, the resulting photoresist at thebottom of stepped portion 110 has a "bridge" between patterns whereexposure is insufficient at reference numeral 6, thereby making itdifficult to form an exact pattern.

To overcome this problem, a multi-layer resist (MLR) method has beenproposed.

FIG. 2 shows a method for forming a fine pattern using the MLR method.

Specifically, a lower photoresist layer 20 is coated on a semiconductorwafer 26 having a stepped structure 110, and an upper photoresist iscoated on an insulating material layer 22, (e.g., an oxide film)disposed between the upper and lower photoresist layers. Then, a light 1is irradiated onto the semiconductor wafer using mask 2 and projectionlens 3, so that the upper photoresist layer can be exposed as shown in(A) of FIG. 2 (reference numeral 24 is an unexposed portion of the upperphotoresist, reference numeral 25 is an exposed portion of the upperphotoresist). Then, the upper photoresist is developed to form an upperphotoresist pattern 24a as shown in (B). Then, as shown in (C) of FIG.2, the insulating material layer 22 is anisotropically etched using theupper photoresist pattern 24a as an etching mask, to thereby form aninsulating pattern 22a. Lower photoresist 20 is anisotropically etched,to thereby form a lower photoresist pattern 20a.

When a fine pattern is formed on a semiconductor wafer having a steppedstructure using, the MLR method, only the upper photoresist is formed,exposed and developed, and the insulating material layer 22 and thelower photoresist 20 are anisotropically etched using the developedupper photoresist as an etching mask, to thereby form a pattern.Therefore, a photoresist residue cannot be generated on thesemiconductor wafer at the bottom of stepped structure 110.

However, the MLR method is too complicated and productivity is poor,which increases cost. In addition, anisotropic etching can result indefects.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a photolithographymask which can correctly form a fine pattern on a semiconductor waferhaving a stepped region or portion.

It is another object of the present invention to provide a simple methodfor manufacturing the photolithography mask.

To accomplish these and other objects, a photolithography mask isprovided for projecting a pattern onto a semiconductor wafer having astepped structure comprising:

a mask substrate having steps conforming with the stepped structure onthe semiconductor wafer; and

an opaque mask pattern for blocking off light from said mask substratewhere said steps are formed.

It is desirable for the steps of the mask substrate to have a thicknesssubstantially equal to the steps on the semiconductor wafer, multipliedby the square of the magnification of a projection lens used with themask.

Further, it is desirable to form steps of the mask substrate so as tohave a phase difference of 360°±5° when light passes through the steppedand non-stepped regions of the mask substrate.

In addition, the sides of the steps of the mask substrate may be one ofthree types, i.e., perpendicular, staircase having a plurality of steps,and a slope having a predetermined incline.

Another photolithography mask for transferring a pattern onto asemiconductor wafer having a stepped structure is provided, comprising:

a mask substrate;

an optically transparent film pattern formed on a portion of the masksubstrate in conference with the stepped structure on the semiconductorwafer; and

an opaque mask pattern for cutting off light from the substrate and theoptical, transparent film pattern.

Further, a mask is provided for transferring a pattern onto asemiconductor wafer having a stepped portion comprising:

a transparent mask substrate;

an opaque mask pattern formed on the mask substrate; and

an optical transmittivity control film pattern formed on a portion ofthe opaque mask pattern and the mask substrate, and which has astructure corresponding to steps formed on the semiconductor wafer.

It is desirable to use materials such as a photoresist, a thin chrome, athin aluminum, spin-on glass (SOG) or SiO₂, whose optical transmittivityis different from that of the material constituting the mask substrate,for the optical transmittivity control film pattern.

In addition, the thickness of optical transmittivity control filmpattern should selected to avoid having a phase difference of 180°±20°between light passing through an optical transmittivity control filmpattern region and non pattern region on a mask substrate.

To accomplish another object of the present invention, a maskmanufacturing method is provided comprising the steps of:

forming steps in a mask substrate in conformance with a steppedstructure on a semiconductor wafer;

forming an opaque material layer for cutting off light from the masksubstrate where the stepped structure is formed; and

forming an opaque mask pattern by patterning the opaque material layer.

The steps of the mask substrate can be formed by directly etching themain surface of the mask substrate, or by depositing a transparentmaterial, (for example, spin-on glass (SOG)), on the mask substrate andpatterning the resultant material.

In addition, a mask manufacturing method is provided comprising thesteps of:

forming an opaque material layer for cutting off light from atransparent mask substrate;

forming an opaque mask pattern by patterning the opaque material layer;

forming an optical transmittivity control film on the mask substratewhere the opaque mask pattern is formed; and

forming an optical transmittivity control film pattern having astructure that corresponds to the steps on the semiconductor wafer to bepatterned, in a portion of said opaque mask pattern and said masksubstrate.

According to the present invention, the same exposing focus can beformed on the steps and on the semiconductor wafer. A clean and correctpattern can be formed by controlling the amount of exposure radiated tothe steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and other advantages of the present invention willbecome more apparent by describing in detail a preferred embodimentthereof with reference to the attached drawings in which:

FIG. 1 shows a method for forming a pattern by the conventional method;

FIG. 2 shows a conventional method for forming a fine pattern using amulti-layer resist (MLR) method;

FIG. 3 illustrates a projection principle of a basic optical system;

FIG. 4 show a method for forming a fine pattern using a first mask ofthe present invention;

FIG. 5 to FIG. 12 are sectional views illustrating a first embodiment ofa method for manufacturing a first mask according to the presentinvention;

FIG. 13 to 16 are sectional views illustrating a second embodiment of amethod for manufacturing a first mask according to the presentinvention;

FIG. 17 is a sectional views illustrating a third embodiment of a methodfor manufacturing a first mask according to the present invention;

FIG. 18 is a sectional view illustrating a method for forming a finepattern using a second mask of the present invention;

FIG. 19 to FIG. 20 are sectional views illustrating a method for forminga second mask of the present invention; and

FIG. 21 to FIG. 22 are sectional views illustrating a simulation resultof forming a photoresist pattern using a second mask of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be explained in more detail with reference tothe attached drawings.

FIG. 3 shows the projection principle of the basic optical system.First, the principle for forming an image is as follows.

As shown in FIG. 3, a focus of an optical system is indicated as F. Theincidental light parallel to an optical axis from the subject O isrefracted and passes focus F. The light that passes the center of theoptical system passes without refraction. A correct image I is thereforeformed at the point where the passed light and the focus-passed lightmeet.

As shown in FIG. 3, if there are arrow subjects 10 and 11 adjacent to anoptical system 9 by the distance of c, corresponding opposite phases 12and 13 are formed on the projection surface according to the aboveprinciples. Arrow subject 10 focuses onto region 12, while arrow subject11 focuses onto region 13 which is offset from region 12 by the distanced. Image size I is determined in accordance with the magnification m ofthe optical system. The magnification can be expressed as follows.##EQU1##

The magnification of the optical systems used for common semiconductorlithography techniques can be 1:1, 1:4, 1:5 or 1:10. Here, in image I,if d is assumed to be the size of a stepped portion on a semiconductorwafer, the subject has a stepped portion with a corresponding size of c.Thus, when steps on a mask are formed proportional to the thickness ofsteps on the semiconductor wafer and a stepped structure on a mask isformed opposite to the stepped structure on the semiconductor wafer,then the same exposing focus can be irradiated onto the semiconductorwafer.

FIG. 4 shows a method for forming a fine pattern using a first mask ofthe present invention manufactured according to the principles describedin FIG. 3.

When an ultraviolet or electron beam, for example, is irradiated on amask 14 having a mask pattern, the mask pattern is projected onto aphotoresist formed on a semiconductor wafer which has steps 110 viaprojection lens 3 of a stepper.

Specifically, as shown in "A" of FIG. 4, when a photoresist is coated ona semiconductor wafer 30 having a stepped portion 110, the photoresistcoating is thicker at the bottom of stepped portion 110 than at the topof stepped portion 110.

As shown in "A" of FIG. 4, when it is assumed that "a" is a line at halfof the thickness of the photoresist at the top of stepped portion 110and "b" is a line at half of the thickness of the photoresist at thebottom of steps, steps corresponding to "a b", i.e., the difference ofthe thickness between the upper end and the lower end of steps, exist onthe semiconductor wafer 30.

The best exposure is achieved when transparent lens 3 focuses at a pointlocated at half of the thickness of the photoresist. Therefore, a maskshould be produced so that "a b", a thickness difference of therespective photoresists of the upper end and the lower end of the step,corresponds to the degree of "A B" on the mask. In this manner, the sameexposing focus is formed on the upper and lower ends of the steppedportion of the semiconductor wafer 30, as described above, to therebyform a correct pattern.

The size of step "A B" on the mask is equal to the size of "a b" on thesemiconductor wafer, in the case of a 1:1 transparent lens, and is"25×(a b)", for the case of a 1:5 transparent lens 3. That is, it isdesirable for the thickness of "A B", i.e., the step on the mask, toequal "step (a b)×(magnification of the transparent lens 2)".

As will be appreciated by those of skill in the art, the step borderregion (reference designation p in FIG. 12) on the mask acts as achromeless phase shifting mask. Thus, a thin line may be produced on thesemiconductor wafer due to the phase difference of the light in the stepand non-step regions on the mask. Therefore, the step on the mask has tobe controlled so that the phase difference is not 180°. The thickness ofthe step necessary to result in a phase difference of 360°, not 180°,can be expressed as follows: ##EQU2##

This expression can be stated in another way. ##EQU3##

Wherein, if 2π is substituted for a phase difference, the result will beexpressed as follows. ##EQU4##

Wherein, λ is the wavelength of the light source in use, n is therefractive index of the mask, and t is step thickness. That is, ift=λ/(n-1), the wavelength of the light source in use is 365 nm, andquartz (whose refractive index n is 1.5) is used as a mask substrate,then, ##EQU5##

That is, mask steps having a thickness of 730 nm must be formed when themask is needed to have a phase difference of 360° rather than 180°.

When a stepper using a transparent lens having a magnification of 1:5 isused, the step thickness on a semiconductor wafer can be expressed asfollows. ##EQU6##

That is, if 292 Å-thick steps exist on the semiconductor wafer 30, aphase difference of 360° exists when steps of 730 nm(7,300 Å) are formedon the mask. In this manner, the chromeless phase shifting mask effectcan be eliminated and exposure focus on the non-stepped regions of thewafer 30 can be improved. As will be appreciated by one skilled in theart, a phase difference of 360° is most desirable in order to avoid thechromeless phase shifting mask effect. However, the chromeless phaseshifting mask effect can be at least reduced, though not perfectlyeliminated, if the phase difference is not 2πn±20° (where n is aninteger).

Accordingly, when a photoresist is exposed using a mask of the presentinvention, an exposure focus is formed at the line of a in thephotoresist at the top of stepped region 110, and an exposure focus isformed at the line of b in the photoresist at the non-step region. Thus,patterning is performed cleanly, without leaving a photoresist residueon the wafer region after developing. By way of example, light such asg-ray (wavelength 436 nm), i-ray (wavelength 365 nm), h-ray (wavelength405 nm), broad band (wavelength 300 to 500 nm, especially 240 to 300nm), KrF excimer laser (wavelength 248 nm) and ArF excimer laser(wavelength 193 nm) can be used as a light source for the exposureprocess. The photoresist should be selected to correspond with thewavelength of the selected light source.

In addition, the step size on the semiconductor wafer 30 can bedetermined within the range which the field depth of a projection lenspermits. Therefore, a thickness "A-B" of a step on the mask has to beformed within the range that satisfies the following expression.##EQU7##

FIG. 5 to FIG. 12 are sectional views illustrating a first embodiment ofa method for manufacturing a first mask of the present invention.

FIG. 5 shows a step of coating photoresists 17 and 17a on a masksubstrate 15 and exposing the resultant structure with opticalradiation.

Photoresists 17 and 17a, having a thickness of between 0.3μm to 2.0μm,are coated onto mask substrate 15 which consists of material transparentwith respect to optical radiation 1, such as quartz. Then, the resultantstructure is exposed, to thereby expose photoresist 17a, predeterminedportion of which is to etch mask substrate 15.

FIG. 6 shows a step of etching the main surface of the substrate 15after photoresist pattern 17 is formed.

More particularly, photoresist pattern 17 is formed by developing theexposed photoresist. Then, the main surface of mask substrate 15 isanisotropically etched using pattern 17 as an etching mask.

FIG. 7 shows a step of stripping photoresist pattern 17 and of removingthe result.

Specifically, mask substrate 15 having a predetermined step portion isformed by removing photoresist pattern 17.

FIG. 8 shows a step of forming an opaque material layer 18 on the masksubstrate 15 having the step portion.

More particularly, opaque material layer 18 is formed by depositing anopaque material for cutting off light, such as chrome, onto masksubstrate 15 where the step portion is formed. At this time, a chromeoxide film can additionally be deposited on the chrome so as to formopaque material layer 18.

FIG. 9 shows a step of coating a photoresist on opaque material layer 18and exposing the resultant material.

Specifically, a photoresist is coated on opaque material layer 18, andis exposed by irradiating light 1. Thus, the photoresist can be dividedinto the exposed photoresist 19a and the unexposed photoresist 19.

FIG. 10 shows a step of patterning the photoresists.

Specifically, a photoresist pattern 20 is formed by developingphotoresists 19 and 19a. Exposed photoresist portions 19a are removed.

FIG. 11 shows a step of forming a opaque mask pattern 18a by patterningopaque material layer 18.

In more detail, opaque material layer 18 is anisotropically etched usingphotoresist pattern 20 as an etching mask. Thus, an opaque mask pattern18a is formed on the substrate 15.

FIG. 12 shows a step of stripping photoresist pattern 20 and removingthe result.

In more detail, opaque mask pattern 18a consisting of opaque material isformed by removing photoresist pattern 20, to thereby complete a mask ofthe present invention. Reference designation P denotes a step borderregion between the step region and the non-step region described above.

FIG. 13 to FIG. 16 are sectional views illustrating a second embodimentof a method for manufacturing a first mask of the present invention.

FIG. 13 shows a step of depositing photoresists 17 and 17a on masksubstrate 15 and exposing the resultant structure.

First, as explained in reference to FIG. 5, photoresists 17 and 17a arecoated onto mask substrate 15. Then, a predetermined portion of thephotoresist is exposed by irradiating light 1. At this time,photoresists 17 and 17a can be deposited at a thickness of approximately2μm to 3μm. Alternatively, photoresists 17 and 17a having a largeincline for a sloped etching due to the characteristics of the materialcan be used.

FIG. 14 shows a step of performing a slope etching on mask substrate 15using a photoresist pattern 21.

Specifically, a photoresist pattern 21 having a slope is formed byetching the exposed photoresist 17a. At this time, as a method forforming the sloped photoresist shown in FIG. 13, a photoresist having alarge slope due to the characteristics of the constituent material canbe used to perform a sloped etching. Otherwise, the photoresist isperpendicularly patterned and the baking temperature is increased toabove the transition temperature of the material constituting masksubstrate 15, to thereby make the photoresist fluid and sloping.

Then, the main surface of mask substrate 15 is slope-etched byperforming an anisotropical etching where photoresist pattern 21 havingthe above-described slope is used as an etching mask. In another method,a perpendicular step is formed in the mask substrate as shown in thefirst embodiment, and a transparent insulating film is deposited. Then,the transparent insulating film is etched so as to form a transparentinsulating film spacer in the side wall of the perpendicular step of themask substrate. Then, the slope-etching can be performed using a slopeof the transparent insulating film spacer.

FIG. 15 shows the step of stripping photoresist pattern 21 and removingthe result.

Specifically, photoresist pattern 21 used as an etching mask is removed,to thereby form mask substrate 15 having a predetermined step.

FIG. 16 shows the step of forming a opaque material layer 22 on masksubstrate 15 having the above-described step.

In more detail, material which can block light, for example, chrome, isdeposited on mask substrate 15 where a step is formed, to thereby formopaque material layer 22.

According to the above-described second embodiment, an anisotropicaletching process is not necessary to ensure a 360° phase difference ofthe step of a mask substrate as in the first embodiment. Rather, asloped etching is performed on the surface of the mask substrate toavoid depending on the thickness of a step of the mask.

FIG. 17 is a sectional view illustrating a third embodiment of a methodfor manufacturing a first mask of the present invention.

Specifically, the border region of the step portion in the masksubstrate is formed into a multi-staircase shape to thereby avoiddepending on the thickness of a step of the mask, as in the firstembodiment.

In addition, as shown in embodiments 1, 2 and 3, the step of a mask isformed as follows.

Material transparent to light, such as spin-on glass (SOG), is coated ona mask substrate without etching the mask substrate. Then, the opticallytransparent film is etched by a photo-etching process, to thereby form astep consisting of an optical transparent film pattern on the masksubstrate.

FIG. 18 illustrates a method for forming a fine pattern using a secondmask of the present invention.

When light, such as ultraviolet or electron beam radiation, isirradiated onto mask 2 having a mask pattern, the mask pattern isprojected onto a photoresist layer formed on a semiconductor wafer 42,which has stepped portion 110, through projection lens 3 of a stepper.

An optical transmittivity control film pattern 30, having a structurecorresponding to a step on the semiconductor wafer 42 to be patterned,is formed from a material whose optical transmittivity is different fromthat of the material of mask substrate 2 with respect to light 1. Theamount of light passing through pattern region C on mask substrate 2should be less than the amount of light which passes through patternregion D, which does not include control transmittivity control filmpattern 30. Thus, a correct pattern can be formed on semiconductor wafer42 having a step portion 110. The optical transmittivity control filmpattern 30 should have a thickness which avoids a phase difference ofbetween 180°±20° when the light passes through a region including thetransmittivity control film pattern and a region which does not includethe transmittivity control film pattern on a mask substrate.

Specifically, after a photoresist layer is deposited on a semiconductorwafer 42 having step portion 110, the photoresist layer is exposed usinga mask 2 upon part of which optical transmittivity control film pattern30 is formed. The photoresist is then divided into an exposedphotoresist portion 32 and an unexposed photoresist portion 31, suchthat a photoresist portion d on the semiconductor wafer 42 (See FIG.18(a)) is exposed via non-pattern region D of the mask 2. Similarly,photoresist portion c on step portion 110 of semiconductor wafer 42 isexposed through pattern region C of the mask 2. Thus, the amount oflight irradiated onto the semiconductor wafer 42 itself is more thanthat irradiated onto the step portion 110 of semiconductor wafer 42. Asa result, the problem of incomplete pattern formation due to aninsufficient amount of light exposure can be solved, and a correctphotoresist pattern 31a can be formed.

FIG. 19 and FIG. 20 are sectional diagrams for illustrating a method formanufacturing a second mask of the present invention.

FIG. 19 shows a step of forming optical transmittivity control films 40and 40a on mask substrate 15a on which an opaque mask pattern 15 isformed, and exposing the formed optical transmittivity control filmswith a light 1.

Specifically, an opaque material for cutting off light, such as chrome,is formed on mask substrate 15, which consists of a material transparentto light, such as quartz. The opaque material is patterned to therebyform an opaque mask pattern 15. Then, optical transmittivity controlfilms 40 and 40a, whose optical transmittivity are different from thatof the material of mask substrate 15, such as a photoresist, aredeposited all over mask substrate 15 where the opaque mask pattern 15 isformed. Then, a predetermined portion 40 of the photoresist is exposedto light 1.

FIG. 20 shows a step of forming transmittivity control film pattern 40a.

Specifically, optical transmittivity control film pattern 40a consistingof a photoresist material is formed on the mask substrate 15a by meansof etching the exposed photoresist material 40. Alternately thephotoresist material can be replaced by a thin layer of chrome,aluminum, spin-on glass (SOG) or SiO₂. When spin-on glass (SOG) is used,spin-on glass (SOG) is deposited on the mask substrate by a spin-coatingmethod, and a photoresist is deposited on the spin-on glass (SOG). Then,after the photoresist is exposed and developed, the spin-on glass (SOG)is anisotropically etched using the patterned photoresist as an etchingmask to thereby form a pattern of spin-on glass (SOG) on the masksubstrate.

FIG. 21 and FIG. 22 are a plan view and a sectional view, respectively,showing a simulation of forming a photoresist pattern using a secondmask of the present invention.

Specifically, a photoresist for forming a pattern is deposited in alayer approximately 1μm thick onto a semiconductor wafer having a stepportion. Then, an i-ray light source (wavelength 365 nm) is irradiatedat a dose of 95μC. The photoresist is then exposed and developed.

Specifically, 80% of the light exposure amount is irradiated onto thestep of semiconductor wafer while 100% of the light exposure amount isirradiated onto semiconductor wafer. Thus, the problem of incompletepattern formation can be solved, to thereby form a correct pattern.

Accordingly, when a new mask of the present invention is used, the sameexposure focus can be irradiated onto a semiconductor wafer and astepped portion formed thereon. Thus, a relatively clean and correctpattern can be formed by controlling the amount of exposure irradiatedonto a step and a semiconductor wafer.

In addition, a simple SLR (single-layer resist) method having can beused instead of a complicated MLR method, thereby simplifying theprocess and in reducing costs. Further, the reliability of asemiconductor device can be greatly improved with a reduction ofmanufacturing failure and poor yield.

The foregoing description is intended to be merely illustrative, andshould not be interpreted as limiting the scope of the invention. It isto be understood that changes and variations may be made withoutdeparting from the spirit or scope of the following claims.

What is claimed is:
 1. A mask for projecting a pattern through aprojection lens onto a semiconductor wafer having a base portion and astepped portion thereon, the mask comprising:a transparent masksubstrate having a stepped portion and a base portion, wherein said masksubstrate is thicker at said stepped portion than at said base portion,said stepped portion on said mask substrate corresponding to the steppedportion on the semiconductor wafer, wherein light passed through saidmask substrate at said stepped portion differs in phase from lightpassed through said mask substrate at said base portion by approximatelyan integral multiple of 2π radians; and an opaque mask pattern formed onsaid mask substrate and arranged to block light from said masksubstrate.
 2. A mask according to claim 1, wherein said stepped portionon said mask substrate has a thickness substantially equal to athickness of the stepped portion on the semiconductor wafer, multipliedby the mathematical square of a magnification of the projection lens. 3.A mask according to claim 1, wherein light passed through said masksubstrate at said stepped portion differs in phase from light passedthrough said mask substrate at said base portion by 360°±5°.
 4. A maskfor projecting a pattern onto a semiconductor wafer having a baseportion and a stepped portion provided thereon, the mask comprising:atransparent mask substrate; a transparent film pattern formed on aportion of said mask substrate so as to define a base portion and astepped portion on said mask substrate, said stepped portion beingthicker than said base portion, said stepped portion on said masksubstrate corresponding to the stepped portion on the semiconductorwafer; and an opaque mask pattern formed on said mask substrate havingsaid transparent film pattern formed thereon, said opaque mask patternblocking light from said mask substrate and said transparent filmpattern, wherein light passed through said mask substrate and saidstepped portion defined by said transparent film pattern differs inphase from light passed through said mask substrate and said baseportion defined said transparent film pattern by approximately anintegral multiple of 2π radians.
 5. A mask according to claim 4, whereinsaid transparent film pattern comprises spin-on glass (SOG).
 6. A maskaccording to claim 1, wherein said stepped portion on said masksubstrate has a sidewall which is substantially perpendicular to anupper surface of said base portion on said mask substrate.
 7. A maskaccording to claim 1, wherein said stepped portion on said masksubstrate has a sidewall which includes a plurality of substep portions.8. A mask according to claim 1, wherein said stepped portion on saidmask substrate has a sidewall which is sloped relative to an uppersurface of said base portion on said mask substrate.
 9. A method formanufacturing a mask for projecting a pattern through a projection lensonto a semiconductor wafer having a stepped portion and a base portionformed thereon, the method comprising the steps of:forming a steppedportion and a base portion on a transparent mask substrate, the steppedportion on the mask substrate corresponding with the stepped portion onthe semiconductor wafer; and forming an opaque mask pattern on the masksubstrate, wherein light which passes through the mask substrate at thestepped portion differs in phase from light which passes through themask substrate at the base portion by approximately an integral multipleof 2π radians.
 10. A method for manufacturing a mask according to claim9, wherein said step of forming the stepped portion and the base portionon the mask substrate is performed by etching the mask substrate.
 11. Amethod for manufacturing a mask according to claim 9, wherein said stepof forming the stepped portion and the base portion on the masksubstrate is performed by depositing a transparent material onto themask substrate and thereafter patterning the transparent material.
 12. Amethod for manufacturing a mask according to claim 9, further includinga step of making a sidewall of the stepped portion formed on the masksubstrate substantially perpendicular to an upper surface of an adjacentbase portion on the mask substrate by anisotropically etching the masksubstrate.
 13. A method for manufacturing a mask according to claim 9,wherein said step of forming the stepped portion on the mask substrateincludes forming the stepped portion on the mask substrate so as to havea thickness substantially equal to a thickness of the stepped portionformed on the semiconductor wafer, multiplied by a magnification of theprojection lens.
 14. A method for manufacturing a mask according toclaim 9, wherein said step of forming the stepped portion on the masksubstrate includes a step of providing the stepped portion formed on themask substrate with a sloped sidewall using a sloped etching method. 15.A method for manufacturing a mask according to claim 14, wherein saidstep of providing the stepped portion formed on the mask substrate witha sloped sidewall is performed by slope-etching using a photoresistmaterial.
 16. A method for manufacturing a mask according to claim 14,wherein said step of providing the stepped portion formed on the masksubstrate with a sloped sidewall comprises the steps of forming a slopein a photoresist, heating the photoresist to a temperature higher than atransition temperature of the mask substrate, and performing a slopeetching according to a slope of the photoresist.
 17. A method formanufacturing a mask according to claim 14, wherein said step ofproviding the stepped portion on the mask substrate with a slopedsidewall comprises the steps of forming perpendicularly shaped steppedportions on the mask substrate, depositing a transparent film over themask substrate having the perpendicularly shaped stepped portions,forming a transparent film spacer on respective side walls of theperpendicularly shaped stepped portions by etching back the transparentfilm, and performing sloped etching using a slope of a respectivetransparent film spacer.
 18. A method for manufacturing a mask accordingto claim 9, wherein said step of forming an opaque mask patterncomprises forming and patterning a chrome material layer.
 19. A methodfor manufacturing a mask according to claim 9, wherein said step offorming an opaque mask pattern comprises forming and patterning a chromeoxide layer.
 20. A mask for projecting a pattern through a projectionlens onto a semiconductor wafer having a stepped portion formed thereon,the mask comprising:a transparent mask substrate; an opaque mask patternformed on said mask substrate; and an optical transmittivity controlfilm pattern formed on at least a part of said opaque mask pattern andsaid mask substrate, said optical transmittivity control film patterndefining a stepped portion and a base portion, wherein said steppedportion on said mask substrate corresponds to the stepped portion on thesemiconductor wafer, wherein light passed through said stepped portiondefined by said optical transmittivity control film pattern differs inphase from light passed through said base portion defined by saidoptical transmittivity control film pattern by approximately an integralmultiple of 2π radians.
 21. A method for manufacturing a mask forprojecting a pattern through a projection lens onto a semiconductorwafer having a stepped portion formed thereon, the method comprising thesteps of:forming an opaque mask pattern on a transparent mask substrate;forming an optical transmittivity control film on the mask substrate onwhich the opaque mask pattern is formed: and forming an opticaltransmittivity control film pattern by patterning the opticaltransmittivity control film, thereby defining a stepped portion and abase portion, the stepped portion defined by the optical transmittivitycontrol film pattern corresponding to the stepped portion formed on thesemiconductor wafer, wherein light passed through the stepped portiondefined by the optical transmittivity control film pattern differs inphase from light passed through the base portion defined by the opticaltransmittivity control film pattern by approximately an integralmultiple of 2π radians.
 22. A method according to claim 21, wherein themask substrate and the optical transmittivity control film pattern aremade from different materials.
 23. A method according to claim 22,wherein the optical transmittivity control film pattern is made from oneof a photoresist, chrome, aluminum, and a spin-on glass, and wherein themask substrate is made of quartz.